1. Field of the Invention
This invention relates to the field of semiconductor devices, and more specifically, to a method to extend the operating life of interconnect lines on integrated circuits.
2. Background Information
In the fabrication of integrated circuits, it is important to form interconnect lines that are able to withstand high current densities. High current densities are due to device scaling, as devices become smaller and incorporate more transistors per unit area, the metal lines which interconnect the transistors (interconnect lines) must be reduced in size, thus increasing the current density within the interconnect lines. The failure rate of interconnect lines has been found to increase as the current density in the interconnect line increases. The increased failure of interconnect lines is due, in large part, to electromigration.
Electromigration is the motion of ions of a conductor, such as aluminum, in response to the passage of current through it. As an example, in an aluminum interconnect line (interconnect line), the electrons flowing through the interconnect line, at high current densities, drag some of the aluminum atoms along the direction of flow. The migrating aluminum atoms produce voids in the upstream end of the interconnect line. The formation of voids results in an increase in the resistance of the interconnect line and eventually causes circuit malfunction. As the electrons drag more and more aluminum atoms down the interconnect line, the voids become larger and larger. Voids may ultimately grow to a size that results in an open-circuit and cause failure of the interconnect line.
FIG. 1a illustrates an interconnect line 100 subjected to a high density current. After a brief period, the current flow 110 pulls metal atoms downstream forming void 120 in the upstream portion of the interconnect line. FIG. 1b illustrates interconnect line 100 of FIG. 1a after a longer period of time. Void 120 has become larger as more metal atoms have been pulled downstream by current flow 110. FIG. 1c illustrates interconnect line 100 of FIG. 1b after an extended period of time. Electromigration has caused void 120 to grow to the full-width of interconnect line 100, thus forming an open circuit and stopping the current flow. This phenomena is referred to as electromigration failure.
The electromigration failure rate of an interconnect line is increased when the current density in the interconnect line is increased. As voids form the resistance and the current density of the interconnect line increase, thereby increasing the electromigration failure rate. Additionally, thinning of the interconnect lines as they cross steep steps in the underlying topography of a semiconductor device accelerates electromigration failure rates. In the case of thinning of interconnect lines, the electromigration failure rate increases because the current density at such a location along an interconnect line increases.
The maximum current at which the interconnect line resistance is stable is called the threshold current. For short interconnect lines, approximately 30 microns or less, the backpressure which builds up as more and more metal atoms are carried downstream is enough to prevent further electromigration at fairly high current densities. Backpressure is the pressure exerted back up the interconnect line which stops the flow of any more metal atoms downstream because the number of metal atoms downstream has already reached saturation. Thus, if no more metal atoms can flow downstream void formation stops and the interconnect line will have reached a steady-state. Once the interconnect line reaches a steady-state, the resistance of the interconnect line remains the same (stabilizes) and the current density at this stage is the threshold current of the interconnect line. The larger the backpressure the higher the threshold current the interconnect line can withstand.
For long interconnect lines, approximately 300 microns or more, backpressure does not build up enough to prevent electromigration. In a long interconnect line there are not enough metal atoms available that can be carried downstream to build a backpressure sufficient enough to stop electromigration. If a sufficient backpressure cannot be built up, a steady-state cannot be reached. Thus, voids form and eventually cause circuit malfunction.
Presently there are several techniques which help to reduce the electromigration failure rate of interconnect lines. The first technique is the addition of electromigration resistant metals to the interconnect structure. For example, small amounts of copper can be added to an aluminum interconnect line. While this technique improves the resistance of the interconnect line to electromigration, it only improves it to a certain degree. An aluminum/0.5% copper interconnect line has am improved electromigration lifetime on the order of 10.times.that of a pure aluminum interconnect line.
A second technique is the construction of refractory metal films in parallel and in contact with the interconnect line, above or below it. Refractory metal films used in parallel with interconnect lines are also known as "shunt layers". For example, a thin titanium or titanium nitride film can be deposited above, below, or in the middle of an interconnect line. When voids are formed in the interconnect line, the current is forced to flow through the parallel refractory metal film. The metal atoms of the refractory metal film do not migrate, thus the refractory metal film does not suffer electromigration failure. Because the refractory metal film has a much higher electrical resistance than the interconnect line there is still the problem of increased resistance. Thus, even with this technique the increase in resistance due to voids causes an increase in the overall resistance of the interconnect line and may eventually cause circuit malfunction.
A third technique is to planarize the intermetal dielectric to eliminate the thinning of interconnect lines as they cross steep steps in the underlying topography of a semiconductor device. This technique decreases the current density where it would otherwise have been increased due to thinner regions of the interconnect line. However, this method does not solve the problem where the current density is increased for reasons other than the thinning of the interconnect line over steep steps.
A fourth technique is to widen the interconnect line to accommodate larger currents. One problem with this technique is that the semiconductor industry is moving toward smaller device characteristics, but wider interconnect lines require more space making the chip larger, slower, and more costly.
Thus, what is needed is a device to prevent void formation in the current path of an interconnect line thereby reducing the effects of electromigration and decreasing the failure rate of interconnect lines.